OM4
A fine, very plastic ball clay · Bodies, casting slips and glazes · Commonly up to about a fifth of a body · A cream buff when fired, not white
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OM#4 is a ball clay: a fine, very plastic clay that goes into a body for workability and dry strength, and into a glaze to hold the slurry together. It fires to a cream buff rather than to white.
OM#4 is a ball clay: fine, very plastic, and what makes a body workable. A porcelain or white stoneware built on kaolin alone is short and tears on the wheel, and a share of ball clay is what makes it throw. It goes into casting slips for the same reason, and into a glaze at a small share, where its fineness keeps the heavier materials in suspension. Most bodies carry it at up to about a fifth, since it brings drying shrinkage along with the plasticity. A body that bloats or comes out with a dark core has carbon in it still burning after the surface has sealed, which is a bisque-firing matter more than a recipe one.
